Simple methods to smooth out weight paints?
Starting from this situation, 



You can use the smooth tool :




Go in weight paint mode
Activate vertex selection (the little icon on the bottom of the screen indicated by a cube with one vertex in yellow)
Select your vertices A
Then in the weight paint tools panels click on 'smooth'
One done, go to the operator options and select 'all groups'
Then tune the amount of smooth you want (or click on the smooth button several times)


You need to go to vertices selection mode because the smooth operates only on a selection (unavailable if there is no selection).

By the way, few other things :


3.6k is not so much at all in general for skinning. But yes, that can be a lot for this particular model
Automatic weights generally does a very good job and I am surprised it does not work here ? Maybe because of your bone structure as the root bone is not placed at the root of the hair ?


What I obtain with 'automatic weights' :



Alternative smoothing using blur brush :